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2011-03-19

〔★ 原発NEWS〕 「石棺」逆効果論

 ロサンゼルス・タイムズ紙が「石棺」化に対する否定的な見方を紹介している。
⇒ http://www.latimes.com/news/nationworld/world/la-sci-japan-quake-nuclear-20110319,0,3085957.story?page=2

 チェルノブイリと違って「フクシマ」には格納容器があり、それを上からコンクリート(と砂)で埋めてしまうことは、逆に格納容器の損傷につながり、逆効果を招きかねないという議論だ。

 (使用済み燃料プールを破壊してしまうというリスクもある。この点にも、以下の記述にそれらしき箇所がある。しかし、石棺化も無理だとしたら……いったい??)

 In the long run, some experts believe the Fukushima plant will have to be entombed. Within about a month of the Chernobyl accident, workers began dumping more than 5,000 tons of sand and concrete on the burning reactor to snuff out the flames and prevent further release of radiation.

But there are major differences between the two facilities that could make it very difficult to carry out a similar operation in Japan.

Chernobyl's damaged reactor did not have a containment vessel, but was exposed to the open air. It was thus imperative to encase the plant in concrete to prevent further escape of radioactive ash.

The reactor cores at Fukushima, in contrast, are housed inside containment vessels made of steel and concrete. One of those may have been cracked, but the evidence so far suggests the cracking is modest, at worst.

Ironically, attempts to blanket those reactors with concrete at this point could wind up damaging the containment vessels more severely, cracking them open and allowing more radiation to escape, experts said.

Similarly, the spent fuel pool for reactor No. 4 may have run dry and may have holes in its side walls. Plus, the pond is 70 feet above the ground. Attempting to encase it in concrete could create massive problems.

Either workers would have to build some type of structure to support the concrete tomb or run the risk that the added weight of any extra concrete and sand piled onto the building would cause the top floors to crumble, allowing the fuel rods to break free.

Encasing the fuel rods in sand and concrete, furthermore, would trap heat and allow them to get much hotter than they are now, potentially enabling them to burn through the concrete and escape from their tomb.

Because the fuel rods and the reactors are so hot, the concrete would undergo a phenomenon called flash-set, in which it would solidify extremely rapidly.

But the result would be a material with a consistency much like gravel rather than stone.

"My gut feeling is, it's a bad idea," Lewis said.

Officials at Japan's Nuclear and Industrial Safety Agency on Saturday said it was highly unlikely that workers could use massive amounts of sand or cement to smother the super-hot fuel inside the reactors or storage pools at the Daiichi plant any time in the near future.

"We believe it is not a realistic option," said the agency's Hidehiko Nishiyama.

Teruaki Kobayashi of Tokyo Electric Power Co. said the company had not ruled out the entombment strategy. But he added that the possibility was remote unless conditions at the plant changed substantially — for instance, if the fuel rods caught fire and began spewing radioactive ash everywhere.

Posted by 大沼安史 at 11:56 午前 |

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